Admixture Phase Research Articles

Heat Capacity and Thermodynamic Functions of Sr(La1-xLnx)2WO7 Compounds Doped with Samarium and Europium

Samples based on strontium, lanthanum and tungsten with the general formula of Sr(La1-xLnx)2WO7 doped with samarium and europium at 1 and 3 at.% were synthesized by the solid-phase method at temperatures up to 1200 °C. The crystal structure of the samples was confirmed by X-ray powder diffraction. A full-profile refinement of the structure of compounds related to monoclinic syngony with the space group P1121/b was performed. The admixture phase is a compound of the Sr3Ln2W2O12 type with a trigonal system and space group R3-C. Based on the results of structure refinement, the ratio of the main compound and the admixture phase in the samples was determined to introduce corrections during measurements. Using adiabatic calorimetry we measured the heat capacity of the samples and found the thermodynamic functions of main compounds over the range of 5‒320 K. Anomalies were detected in the heat capacity of the samples below 15 K, and we calculated the excess and lattice heat capacity for these anomalies by means of linearization methods.

Synthesis and properties study of the X-ray phosphors Gd2O2S:Tb

Gd2O2S:Tb based X-ray stimulated luminescent phosphors for X-ray intensifying screens, X-ray-visible light converters and bio-imaging are synthesized using two different methods (in a neutral or reducing atmospheres). The effects of atmosphere at synthesis, activator and flux content in the charge mixture and crucible lacing upon the prepared phosphors phase composition and luminescence intensity are studied. The optimal synthesis conditions are determined affording a Gd2O2S:Tb phosphor free of admixture phases and providing the highest luminescence intensity upon excitation with «hard» X-ray irradiation. The synthesized phosphor is characterized by particle size distribution analysis and X-ray stimulated luminescence spectra measurements.

Synthesis and hydration behaviour of calcium zirconium aluminate powders by modifying co-precipitation method

Calcium zirconium aluminate, Ca7ZrAl6O18, is only the ternary compound of CaO, Al2O3 and ZrO2 with hydration reactivity. Calcium zirconium aluminate is conventionally synthesized by a solid-state reaction processing using CaCO3, Al2O3 and ZrO2, as starting materials at temperatures exceeding 1400 °C. In the present study, Ca7ZrAl6O18 along with some admixture phases was prepared by modifying co-precipitation method using ammonia solution, NH3(aq) or saturated oxalate ammonium solution in concentrated ammonia solution, (NH4)2C2O4–NH3(aq) as a precipitation agent. Crystalline Ca7ZrAl6O18 phase was detected at relatively low temperature (~900 °C) together with other intermediate crystalline products. Formation of the final product of Ca7ZrAl6O18 powders, with CaZrO3 and Ca12Al14O33 as accessory phases, requires heat treatment at 1200 °C for 5 h. Two novel approaches for the synthesis of Ca7ZrAl6O18 affect its further hydration behaviour. Ca3 [Al(OH)6]2 or C3AH6 with a hitherto unknown rounded crystal habit was formed within the cement pastes made of Ca7ZrAl6O18, which was synthesized by the modifying co-precipitation method using (NH4)2C2O4–NH3(aq) as a precipitation agent.

Effect of preparation route on Sr2TiO4 catalyst for the oxidative coupling of methane

Four different preparation routes were used to synthesize Sr2TiO4 with the structure of layered perovskite: sol-precipitation (SP), co-precipitation (CO), citrate precursor (CT) and mechanochemical (MA) methods with further calcinations at 1100 °C. The XDR, XPS, BET, SEM techniques showed that the different preparation routes caused the formation of layered perovskite phase with different admixture phases, particle sizes and the surface enrichment with layered perovskite (for SP and MA samples) or strontium oxide (for CO and CT samples). The trend of increasing of CH4 conversion and C2-yield in oxidative coupling of methane at 800-900 °C was found to be the following: SP≈MA > CO > CT samples. The C2-yield of the most active catalysts was measured to be about 12%.

Physico-Chemical Properties of MgGa Mixed Oxides and Reconstructed Layered Double Hydroxides and Their Performance in Aldol Condensation of Furfural and Acetone.

MgGa layered double hydroxides (Mg/Ga = 2–4) were synthesized and used for the preparation of MgGa mixed oxides and reconstructed hydrotalcites. The properties of the prepared materials were examined by physico-chemical methods (XRD, TGA, NH3-TPD, CO2-TPD, SEM, and DRIFT) and tested in aldol condensation of furfural and acetone. The as-prepared phase-pure MgGa samples possessed hydrotalcite structure, and their calcination resulted in mixed oxides with MgO structure with a small admixture phase characterized by a reflection at 2θ ≈ 36.0°. The interaction of MgGa mixed oxides with pure water resulted in reconstruction of the HTC structure already after 15 s of the rehydration with maximum crystallinity achieved after 60 s. TGA-MS experiments proved a substantial decrease in carbonates in all rehydrated samples compared with their as-prepared counterparts. This allowed suggesting presence of interlayer hydroxyls in the samples. Acido-basic properties of MgGa mixed oxides determined by TPD technique did not correlate with Mg/Ga ratio which was explained by the specific distribution of Ga atoms on the external surface of the samples. CO2-TPD method was also used to evaluate the basic properties of the reconstructed MgGa samples. In these experiments, an intensive peak at T = 450°C on CO2-TPD curve was attributed to the decomposition of carbonates newly formed by CO2 interaction with interlayer carbonates rather than to CO2 desorption from basic sites. Accordingly, CO2-TPD method quantitatively characterized the interlayer hydroxyls only indirectly. Furfural conversion on reconstructed MgGa materials was much larger compared with MgGa mixed oxides confirming that Brønsted basic sites in MgGa catalysts, like MgAl catalysts, were active in the reaction. Mg/Ga ratio in mixed oxides influenced product selectivity which was explained by the difference in textural properties of the samples. In contrast, Mg/Ga ratio in reconstructed catalysts had practically no effect on the composition of reaction products suggesting that the basic sites in these catalysts acted similarly in aldol condensation of acetone with furfural. It was concluded that the properties of MgGa samples resembled in a great extent those of MgAl hydrotalcite-based materials and demonstrated their potential as catalysts for base-catalyzed reactions.

Synthesis and properties of binary spinels in a NiO–CuO–Fe2O3–Cr2O3 system

The conditions for the formation of a spinel structure from a NiO–CuO–Fe2O3–Cr2O3 oxide mixture using several technological approaches have been examined. Addition of KCl is accompanied with the formation of two spinel-like phases, whereas in the absence of KCl just one solid solution of nickel–copper ferrite–chromite with the structure of a cubic spinel is formed. At the temperature of thermal treatment of 900°C, the presence of an admixture phase of the delafossite (CuCrO2) type was established. The conditions for the fabrication of samples containing two spinel phases (cubic and tetragonal) characterized with the most developed surface and manifesting = increased catalytic activity in the reaction of the decomposition of an organic substance by hydrogen peroxide have been formulated. The studied features of spinel synthesis can be of interest for developing materials with an active surface promising for application as adsorbents of catalysts and sensors.

Production of mono- and bimetallic nanoparticles of noble metals by pyrolysis of organic extracts on silicon dioxide

In the present work the influence of the tri-n-octylammonium (Oct3NH+) salt anion (PtCl62-, PdCl42-, AuCl4−) nature on the phase composition and mean size of crystallites of the extract pyrolysis products on the SiO2 nanopowder has been studied. The XRD phase analysis of the composites (metal loading 2.4 wt.%) made under the same conditions, at the pyrolysis of Pt- and Au-containing extracts has shown the formation of nanoparticles of Pt (dPt = 15 nm) and Au (dAu = 33 nm), respectively. The end-product of the pyrolysis of the Pd-containing extract has an admixture phase of PdO along with the main metal phase (dPd = 21 nm). At the preparation of bimetallic particles (Pt-Pd, Pt-Au, Pd-Au) on the SiO2 nanopowder it has been found that the nanoparticles of the PtPd alloy, Pt and Au or Pd and Au nanoparticles are the products of the thermal decomposition of two-component mixtures of extracts. The investigation of catalytic properties of the produced composites in the reaction of glycerol oxidation by molecular oxygen in alkaline aqueous solutions has shown that all bimetallic composites exhibit catalytic activity in contrast to monometallic ones.

A glass-ceramic material for fixation of radioactive waste

In this article, a starting mixture for the preparation of glass-ceramic material for radioactive waste (RW), consisting of 85-95 mass % basanite and 5-15 mass % oxides of elements in I-VIII group of the Periodical table of elements imitating RW, is proposed. The glass-ceramic material is obtained by melting the starting mixture in air at 1450?C for 1 hour and by further crystallization of the melts at 950?C for 30 minutes. It has been noticed that the texture of the glass-ceramic material is microgranular. The main mineral is pyroxene, while a mixture phases are magnetite, hematite and residual glass. It was shown that the RW elements are fixed in the pyroxene and partly in the admixture phases.

Calculating the boundary conditions of the formation of solid-phase metal sulfides and selenides by deposition with thio- and selenourea

Calculation equations that allow one to determine the boundary conditions of the formation of solid-phase metal sulfides and selenides with different valences according to the decomposition constants of thio- and selenourea and the kinetic dependence of the accumulation of chalcogenide ions in the reaction system are proposed. The contribution from the crystallization factor to the determination of the boundary conditions of the formation of the solid phase is shown with allowance for the nucleation of metal chalcogenide. The calculation equations determining the boundary conditions of the formation of the main admixture phases upon the hydrochemical deposition of chalcogenides, metal hydroxide, and cyanamide are derived.

Extended synthetic study of ?-type manganese oxide with porous structure

α-type manganese oxide (α-MnO2) was synthesized by pyrolysis of manganese carbonate and potassium t-butoxide. The method is valid even when their molar basis fraction is varied from 0% to 100%. For some potassium butoxide content percentages, the calcined material which is mainly α-MnO2 contained an admixture phase of bixbyite (0–7%) or γ-Mn2O3(11–20%) or both the latter phases (7–11%). The acid-treated materials are single phase α-MnO2 containing various amounts of structural-water. The degree of crystallinity is very high when the material was prepared with a potassium butoxide content of less than 20%, but materials of distorted structure are obtained with over 20%. The materials of high crystallinity contain less than about 0.5 molecules of structural-water per open pore site, but the distorted materials contain about two molecules of the water per site. The degree of ease of deformation, namely, the flexibility of the structure was an index to the amount of ion-exchange uptake on the materials. The possibility of controlling the properties of α-MnO2 ion-exchange material is suggested by changing potassium butoxide content in this synthetic procedure.

Calculation of two-phase flow over a sphere including particle interaction

Within the framework of the kinetic approach to the description of the motion of a gas suspension estimates are obtained for the flow parameters on the basis of which it appears possible to apply the Chapman-Enskog method to an equation of the Boltzmann type written for the admixture phase. In this case the carrier gas can be described within the framework of the ideal gas model with allowance for interaction with the particles of the admixture phase. As the equation of state for the particle “gas” an equation of the van der Waals type is employed. This makes it possible to take regions of high particle concentration into account more correctly than within the framework of the ideal gas model. The problem of supersonic dusty gas flow over a sphere is solved on the basis of this approach.

Phase transition in the system solid–gas admixture due to the macroscopic defect formation and growth

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Relations between the electronic properties and the chemical bonding of sb xBi 2− xTe 3− ySe y system

The electronic properties of Sb x Bi 2− x Te 3− y Se y were investigated. The materials were prepared by melting, quenching and annealing. It is found that y 0, the value of y for which the thermoelectric power reverses from p-type to n-type, coincides with the electrical conductivity minimum, and increases with increasing x. An explanation of the results is proposed based on a model of chemical bonding in compounds of the type A v 2B VI 3. The smaller value of y 0 for x = 0 than for x = 1.0 is interpreted by taking into account the additional effect of the variation of carrier mobility with y. X-ray analysis shows that the system is a continuous solid solution with the hexagonal structure except for the region whose composition is close to Sb 2Se 3. Curves of the electrical conductivity and the thermoelectric power are discontinuous, where the admixture phases of hexagonal and orthorhombic structures are shown.